An electrodynamic acoustic transducer is a device that transforms electrical signals into sound waves, for example, into audible sounds. Its design is an important determinant of overall performance of audio reproduction and generation systems. In choosing a particular acoustic transducer design, engineers generally balance many competing considerations. Such considerations may include frequency range of the transducer, in-band amplitude and phase distortions, efficiency, and the Q factor. Electrodynamic transducers are generally categorized as (1) direct radiating transducers (“direct radiators”) in which the vibrating surface radiates sound waves directly into open air, or (2) horn-loaded transducers that radiate through a horn, i.e., transducers in which a horn is interposed between the vibrating surface and the open air. Horn-loaded transducers are also known as horn-driven transducers and compression drivers.
A typical dynamic transducer/speaker includes an electrodynamic motor that moves a diaphragm or cone. The motor of the transducer has a voice coil with wire windings on a voice coil former. The voice coil moves along a cylindrical pole piece in an air gap where magnetic field (flux) is generated by a permanent magnet. The former of the voice coil is mechanically coupled to the diaphragm. When an electrical current drives the voice coil, the coil moves under influence of the Lorentz electromotive force exerted by the magnetic field of the permanent magnet on the charged particles flowing through the voice coil's windings. The diaphragm moves together with the coil, creating variable acoustic pressure that generates the sound represented by the electrical current.
This design has performance deficiencies at both low and high frequencies. At low frequencies, for example, air tends to leak through the gap between the voice coil and the pole piece, causing noise and loss of acoustic output power. This is particularly problematic in direct radiators, because of the relatively low sound pressure generated by the diaphragm. At high frequencies, the cavity formed by the diaphragm, pole, and voice coil tends to resonate, causing irregularities in the frequency response, i.e., exacerbating sound distortions. Moreover, an underdamped inner surface of the diaphragm may cause unwanted reflections, which further add to the high frequency distortions.
One way to alleviate some of these disadvantages is illustrated in FIG. 1, which shows an electrodynamic acoustic transducer 100. The transducer 100 includes a dustcap 105 that effectively seals the air leaks through a gap 110 between a voice coil 115 and pole piece 120. By sealing the gap 110, the dustcap 105 reduces the low frequency noise and improves acoustic power output of the transducer 100. The dustcap 105 also provides a resistive termination of the inner diaphragm 125, dampening unwanted reflections. This, however, is a partial solution: the dustcap 105 does not fill the cavity formed by the voice coil 115, pole piece 120, and diaphragm 125. Therefore, the dustcap 105 does not eliminate the cavity resonances that tend to distort high frequency response of the transducer 100.
Another way to alleviate some of the disadvantages of the typical transducer design is by using a waveguide extension structure. This is illustrated in FIG. 2, which shows an electrodynamic acoustic transducer 200. The transducer 200 is similar to the transducer 100 of FIG. 1, but without a dustcap. Instead, a waveguide extension structure 250 is disposed within the cavity formed by a voice coil 215, pole piece 220, and diaphragm 225. The waveguide extension structure 250 fills this cavity and reduces the high frequency distortions that result from the cavity resonances. Unfortunately, the waveguide extension structure 250 does not prevent air leakage through a gap 210 between the voice coil 215 and the pole piece 220, and does not provide termination damping.
Thus, known electrodynamic acoustic transducers suffer from one or more of the deficiencies described above. It would be desirable to provide an approach for improving transducer response at both low and high frequencies, reducing noise, and reducing or preventing loss of acoustic power output due to air leakage between a transducer's voice coil and pole piece.